WASHINGTON—Massachusetts Sen.
Elizabeth Warren
on Monday announced the formation of an exploratory committee for president, making her the first top-tier candidate to make a formal move toward running for the 2020 Democratic nomination.

Ms. Warren, long among the highest-profile members of the Democratic Party’s liberal wing, is one of at least two dozen candidates who might seek the party’s presidential nomination. Her political stature within the party has diminished since her October release of a genetic analysis that she said provided evidence of her Native American ancestry. That was received poorly by some Democrats, and President Trump mocked her.

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“Today, I’m launching an exploratory committee for president because the outcome of this election will depend on you,” Ms. Warren says in a four-minute video that includes images of President Trump, Treasury Secretary
Steven Mnuchin
and departing GOP House Speaker
Paul Ryan.
“For the last two years, millions of people have done more than they ever thought they would to protect the promise of America. And here’s what we learned: If we organize together, if we fight together, if we persist together, we can win. We can and we will.”

A federal exploratory committee will allow Ms. Warren to travel the country and raise money from supporters before launching a formal campaign. Exploratory committees let federal candidates raise and spend money to “test the waters” for a campaign without transparency requirements mandated by the Federal Election Commission once a candidate formally declares they are seeking office.

Several other Democrats, including as many as eight senators, are expected to launch campaigns or exploratory committees in January to begin raising money before the first major fundraising deadline on March 31.

Ms. Warren enters a field that includes Maryland Rep.
John Delaney,
who announced his candidacy in 2017, and
Julián Castro,
the former U.S. Housing and Urban Development secretary, who formed an exploratory committee on Dec. 12. Mr. Castro plans to make his candidacy formal at a Jan. 12 event in San Antonio.

California Sen. Kamala Harris has a book due to be released Jan. 8 and is expected to launch her campaign following a brief book tour. Four would-be candidates spoke at a liberal Iowa group’s fundraising dinner two weeks ago. And former
Virginia Gov. Terry McAuliffe
said Sunday that he is considering a White House campaign.

The contours of Democratic presidential field, which is expected to be the party’s largest since 17 candidates ran in 1976, will be shaped by decisions made by former Vice President
Joe Biden
and Texas Rep. Beto O’Rourke. The two are each publicly weighing a run but haven’t yet committed.

Mr. Biden is the consistent polling leader, while
Mr. O’Rourke,
who narrowly lost a 2018 Senate race to Republican Sen. Ted Cruz, has the party’s largest current small-dollar donor list after raising more than $80 million in his Senate race without accepting funds from political action committees.

Neither Mr. Biden nor Mr. O’Rourke has publicly hinted at their timing for making a decision on whether to enter the race.

In a nod to contemporary Democratic voters’ demands that candidates reveal more of their personality, Ms. Warren spent part of New Year’s Eve live-streaming herself discussing her candidacy while drinking a beer from her kitchen in Cambridge, Mass. Mr. O’Rourke, meanwhile, aired video of himself and his wife drinking New Year’s champagne from inside an igloo they built at a New Mexico ski resort.

Ms. Warren, 69 years old, an Oklahoma-born former Harvard Law School professor with a specialty in bankruptcy law, saw her national political stature rise following the 2008 financial crisis. She was the architect of the Consumer Financial Protection Bureau and was President Obama’s first choice to lead it before her nomination was blocked by Senate Republicans.

Ms. Warren instead ran for the Senate in 2012, defeating GOP Sen. Scott Brown. Once she arrived in Washington, Ms. Warren became the most prominent liberal Democrat on the Senate Banking Committee.

Following Mr. Trump’s election, Ms. Warren has repeatedly accused Trump-appointed officials of putting the financial system and consumers at risk in their efforts to ease the regulatory burden on banks. She opposed a banking deregulatory bill supported by several fellow Democrats and signed by President Trump in May.

Ms. Warren has also turned her attention toward large banks, which she has said are now “bigger than they were when they were too big to fail.” She has come down hard against
Wells Fargo
& Co. over its fake-accounts scandal, whose former CEO
John Stumpf
stepped down a month after a contentious congressional hearing during which Ms. Warren told him he “should resign.”

Republican National Committee Chairwoman
Ronna McDaniel
on Monday criticized Ms. Warren as “another extreme far-left obstructionist and a total fraud.” Mr. Trump has often referred to the senator mockingly as “Pocahontas” due to her claim of Native American ancestry. In a Fox News interview Monday, Mr. Trump said, “I wish her well. I hope she does well. I’d love to run against her.”

Ms. Warren, in a Monday news conference outside her home in Cambridge, Mass., didn’t directly address questions about Mr. Trump and whether she is “too polarizing.”

“The problem we’ve got right now in Washington is that it works great for those who have got money to buy influence,” she said. “I’m fighting against that and you bet it’s going to make a lot of people unhappy.”

Ms. Warren foreshadowed Monday’s announcement over the weekend, when she changed her
Twitter
handle to drop the association with Massachusetts, her home state. She now tweets at @ewarren.

But while Ms. Warren is widely known and admired by many Democrats, some within the party questioned the strategy and timing of the release of the genetic study. Members of Native American communities had mixed responses to her move.

In September, a poll of first-in-the-nation Iowa Democrats by the firm Focus on Rural America found Ms. Warren had support of 16% of would-be Iowa caucus-goers—second only to Mr. Biden. By mid-December, Ms. Warren’s support dropped to 9%, putting her in fifth place. She is planning a trip to Iowa this weekend.

Ms. Warren has avoided traveling to early nominating states like Iowa and New Hampshire, which is rare among would-be top 2020 Democratic presidential candidates who spent much of 2018 plotting White House runs.

Ms. Warren finished her successful 2018 Senate re-election bid with $12.5 million on hand, a sum larger than any other would-be presidential candidate has to begin a 2020 campaign. During her Monday news conference, Ms. Warren said she had already received contributions for her presidential campaign from all 50 states, as well as Puerto Rico and Washington, D.C.